Very Confused And Embarrassed

I’ve gotten myself in a horrible situation which has me both embarrassed and very confused.
I’m 27 years old and been married for four years. My husband and I were having a few problems so we separated for a couple of months but with hopes of working things out. Things got better so I moved back home a week and half ago.
However last month while we were separated I was out for an evening with a couple of girlfriends and met a nice guy who I ended up having a one night stand with.
When my period was supposed to come this past Tuesday it didn’t. I waited until yesterday then took a pregnancy test and I’m pregnant. Of course there’s no doubt who knocked me up.
I’m so embarrassed because my husband and I had been trying to have a baby for quite a while with no success.
I’m sure this will be the end of our marriage but now I’m confused as to what to do about being pregnant because it was a one night stand and I have no idea how to find the guy.
I’ve wanted a baby so bad but not like this.